Incorporation by Reference
The disclosure of the following priority application is herein incorporated by reference:
Japanese Patent Application No. 9-035537 filed Feb. 4, 1997.
1. Field of Invention
The present invention relates to a projection exposure apparatus, and more particularly, to a projection exposure apparatus that projects, by exposure, an image of a pattern formed on a mask onto a reactive substrate via a projection optical system and controls the position of the mask by applying force directly to the mask.
2. Description of Related Art
A lithography process typically is used in manufacturing semiconductor devices, liquid crystal devices, and the like. A projection exposure apparatus is used in this lithography process. The projection exposure apparatus illuminates a mask (hereinafter also referred to as a reticle) with a pattern formed thereon and projects by exposure an image of the pattern formed on the mask via a projection optical system onto a wafer or a glass substrate (hereinafter also referred to as a reactive substrate). The reactive substrate is located at a position conjugate with a pattern surface of the mask and coated with a resist.
Known projection exposure apparatus of this type are, for example, (1) a reduction projection exposure apparatus using the step-and-repeat method (called a stepper), and (2) a scanning exposure apparatus using the step-and-scan method. In the step-and-repeat method, a reactive substrate is laid on a substrate stage, which is movable in two dimensions (X and Y directions), and stepped by the substrate stage (stepping). Using this method, a pattern image on a reticle is sequentially projected by exposure onto a plurality of shot regions on the reactive substrate. In the step-and-repeat method, the entire mask pattern is exposed onto a shot region at one time. The step-and-scan method repeats, by the above-mentioned stepping operation, an operation of sequentially projecting a pattern image formed on a reticle onto shot regions on a reactive substrate while relatively scanning the reticle and the reactive substrate with respect to a projection optical system. That is, in the step-and-scan method, the reticle and substrate are both moved (in synchronism) during exposure. In contrast to the step-and-repeat method, the step-and-scan method exposes each shot region piece-by-piece instead of all at once.
In these projection exposure apparatus, for example, a reticle is fixed on a reticle stage, which is movable in the two-dimensional plane, by a vacuum chuck or the like. The position of the reticle stage is measured by projecting an interferometric beam from an interferometer onto a planar reflecting surface (e.g., movable mirror) located on the reticle stage and controlled according to this positional information. As a result, the position of the reticle is indirectly controlled via the reticle stage.
In the above-mentioned conventional projection exposure apparatus, the demands for throughput have become more exacting year after year. Although there has recently been in use, for example, a method of exposing a plurality of chips simultaneously, this causes the size of the shot region to be increased and the size of the reticle is also thereby increased. However, since, as mentioned above, the reticle is mounted on the reticle stage in the conventional projection exposure apparatus, the increase in size of the reticle inevitably results in an increased size of the reticle stage. As a result, the weight of the entire reticle stage, whose position is to be controlled, is increased remarkably. Since F=ma, the increase in size of the reticle is not preferable for position control. In particular, since the scanning projection exposure apparatus repeats operations of accelerating the reticle to the scanning velocity, performing exposure, decelerating, and stopping the reticle, if the weights of the reticle and the reticle stage are increased as mentioned above, the acceleration and deceleration are restricted, and throughput is reduced.
Furthermore, since the reticle is fixed on the reticle stage by a vacuum chuck or the like in the conventional projection exposure apparatus, if the size of the reticle stage is increased, the force applied to the reticle during scanning exposure will be greater, and there is a possibility that the reticle cannot be held by the vacuum chuck and will be displaced.
Further still, when the reticle is fixed by the vacuum chuck or the like, it is prone to be bent by vacuum adsorption, which has an adverse effect on imaging characteristics.
For an example of a system in which a stage is supported by fluid bearings, see U.S. Pat. No. 5,260,580. For examples of systems in which stages are supported by electromagnetic forces, see U.S. Pat. No. 4,485,339 and U.S. Pat. No. 4,506,204.